Cellulosic fibrous structures, such as paper, are well-known in the art. Such fibrous structures are in common use today for paper towels, toilet tissue, facial tissue, etc. To meet the needs of the consumer, these cellulosic fibrous structures must balance several competing interests. For example, the cellulosic fibrous structure must have a sufficient tensile strength to prevent the cellulosic fibrous structure from tearing or shredding during ordinary use or when undue tensile forces are not applied. The cellulosic fibrous structure must also be absorbent, so that liquids may be quickly absorbed and fully retained by the cellulosic fibrous structure. The cellulosic fibrous structure should also exhibit sufficient softness, so that it is tactilely pleasing and not harsh during use. The fibrous structure should exhibit a high degree of opacity, so that it does not appear flimsy or of low quality to the user. Against this backdrop of competing interests, the cellulosic fibrous structure must be economical, so that it can be manufactured and sold for a profit, and yet is affordable to the consumer.
Tensile strength, one of the aforementioned properties, is the ability of the fibrous structure to retain its physical integrity during use. Tensile strength is controlled by the weakest link under tension in the cellulosic fibrous structure. The cellulosic fibrous structure will exhibit no greater tensile strength than that of any region in the cellulosic fibrous structure which is undergoing a tensile loading, as the cellulosic fibrous structure will fracture or tear through such weakest region.
The tensile strength of a cellulosic fibrous structure may be improved by increasing the basis weight of the cellulosic fibrous structure. However, increasing the basis weight requires more cellulosic fibers to be utilized in the manufacture, leading to greater expense and requiring greater utilization of natural resources for the raw materials.
Absorbency is the property of the cellulosic fibrous structure which allows it to attract and retain contacted liquids. Both the absolute quantity of liquid retained and the rate at which the fibrous structure absorbs contacted liquids must be considered with respect to the desired end use of the cellulosic fibrous structure. Absorbency is influenced by the density of the cellulosic fibrous structure. If the cellulosic fibrous structure is too dense, the interstices between fibers may be too small and the rate of absorption may not be great enough for the intended use. If the interstices are too large, capillary attraction of contacted liquids is minimized and, due to surface tension limitations, liquids will not be retained by the fibrous structure.
Softness is the ability of a cellulosic fibrous structure to impart a particularly desirable tactile sensation to the user's skin. Softness is influenced by bulk modulus (fiber flexibility, fiber morphology, bond density and unsupported fiber length), surface texture (crepe frequency, size of various regions and smoothness), and the stick-slip surface coefficient of friction. Softness is inversely proportional to the ability of the cellulosic fibrous structure to resist deformation in a direction normal to the plane of the cellulosic fibrous structure.
Opacity is the property of a cellulosic fibrous structure which prevents or reduces light transmission therethrough. Opacity is directly related to the basis weight, density and uniformity of fiber distribution of the cellulosic fibrous structure. A cellulosic fibrous structure having relatively greater basis weight or uniformity of fiber distribution will also have greater opacity for a given density. Increasing density will increase opacity to a point, beyond which further densification will decrease opacity.
One compromise between the various aforementioned properties is to provide a cellulosic fibrous structure having mutually discrete zero basis weight apertures amidst an essentially continuous network having a particular basis weight. The discrete apertures represent regions of lower basis weight than the essentially continuous network providing for bending perpendicular to the plane of the cellulosic fibrous structure, and hence increase the flexibility of the cellulosic fibrous structure. The apertures are circumscribed by the continuous network, which has a desired basis weight and which controls the tensile strength of the fibrous structure.
Such cellulosic structures are known in the prior art. For example, U.S. Pat. No. 3,034,180 issued May 15, 1962 to Greiner et al. discloses cellulosic fibrous structures having bilaterally staggered apertures and aligned apertures. Moreover, cellulosic fibrous structures having various shapes of apertures are disclosed in the prior art. For example, Greiner et al. discloses square apertures, diamond-shaped apertures, round apertures and cross-shaped apertures.
However, apertured cellulosic fibrous structures have several shortcomings. The apertures represent transparencies in the cellulosic fibrous structure and may cause the consumer to feel the structure is of lesser quality or strength than desired. The apertures are generally too large to absorb and retain any liquids, due to the limited surface tension of liquids typically encountered by the aforementioned tissue and towel products. Also, the basis weight of the network around the apertures must be increased so that sufficient tensile strength is obtained.
In addition to the zero basis weight apertured degenerate case, attempts have been made to provide a cellulosic fibrous structure having mutually discrete nonzero low basis weight regions amidst an essentially continuous network. For example, U.S. Pat. 4,514,345 issued Apr. 30, 1985 to Johnson et al. discloses a fibrous structure having discrete nonzero low basis weight hexagonally shaped regions. A similarly shaped pattern, utilized in a textile fabric, is disclosed in U.S. Pat. 4,144,370 issued Mar. 13, 1979 to Boulton.
The nonapertured structures disclosed in these references provide the advantages of slightly increased opacity and the presence of some absorbency in the discrete low basis weight regions, but do not solve the problem that very little tensile load is carried by the discrete nonzero low basis weight regions, thus limiting the overall burst strength of the cellulosic fibrous structure. Also, neither Johnson et al. nor Boulton teach cellulosic fibrous structures having relatively high opacity in the discrete low basis weight regions.
Cellulosic fibrous structures are usually manufactured by depositing a liquid carrier having cellulosic fibers homogeneously entrained therein onto an apparatus having a fiber retentive liquid pervious forming element. The forming element may be generally planar and is typically an endless belt.
The aforementioned references, and additional teachings such as U.S. Pat. No. 3,322,617 issued May 30, 1967 to Osborne; U.S. Pat. No. 3,025,585 issued Mar. 20, 1962 to Griswold, and U.S. Pat. No. 3,159,530 issued Dec. 1, 1964 to Heller et al. disclose various apparatuses suitable for manufacturing cellulosic fibrous structures having discrete low basis weight regions. The discrete low basis weight regions according to these teachings are produced by a pattern of upstanding protuberances joined to the forming element of the apparatus used to manufacture the cellulosic fibrous structure. However, in each of the aforementioned references, the upstanding protuberances are disposed in a regular, repeating pattern. The pattern may comprise protuberances staggered relative to the adjacent protuberances or aligned with the adjacent protuberances. Each protuberance (whether aligned, or staggered) is equally spaced from the adjacent protuberance. Indeed, Heller et al. utilizes a woven Fourdrinier wire for the protuberances.
The arrangement of equally spaced protuberances represents another shortcoming in the prior art. The apparatuses having this arrangement provide substantially uniform and equal flow resistances (and hence drainage and hence deposition of cellulosic fibers) throughout the entire liquid pervious portion of the forming element utilized to make the cellulosic fibrous structure. Substantially equal quantities of cellulosic fibers are deposited in the liquid pervious region because equal flow resistances to the drainage of the liquid carrier are present in the spaces between adjacent protuberances. Thus, fibers will be relatively homogeneously and uniformly deposited, although not necessarily randomly or uniformly aligned, in each region of the apparatus and will form a cellulosic fibrous structure having a like distribution and alignment of fibers.
One teaching in the prior art not to have each protuberance equally spaced from the adjacent protuberances is disclosed in U.S. Pat. No. 795,/719 issued Jul. 25, 1905 to Motz. However, Motz discloses protuberances disposed in a generally random pattern which does not advantageously distribute the cellulosic fibers in the manner most efficient to maximize any one of or optimize a majority of the aforementioned properties.
Accordingly, it is an object of this invention to overcome the problems of the prior art and particularly to overcome the problems presented by the competing interests of maintaining high tensile strength, high absorbency, high softness, and high opacity without unduly sacrificing any of the other properties or requiring an uneconomical or undue use of natural resources. Specifically, it is an object of this invention to provide a method and apparatus for producing a cellulosic fibrous structure, such as paper, by having multiple and different flow resistances to the drainage of the liquid carrier of the fibers in the apparatus.
By having regions of relatively high and relatively low resistance to flow present in the apparatus, one can achieve greater control over the orientation and pattern of deposition of the cellulosic fibers, and obtain fibrous structures not heretofore known in the art. Generally, there in an inverse relation between the flow resistance of a particular region of the liquid pervious fiber retentive forming element and the basis weight of the region of the resulting cellulosic fibrous structure corresponding to such regions of the forming element. Thus, regions of relatively low flow resistance will produce corresponding regions in the cellulosic fibrous structure having a relatively high basis weight and vice versa.
More particularly, the regions of relatively low flow resistance should be continuous so that a continuous high basis weight network of fibers results, and tensile strength is not sacrificed. The regions of relatively high flow resistance (which yield relatively low basis weight regions in the cellulosic fibrous structure) may either be discrete or continuous, as desired.
According to the present invention, the forming element is a forming belt having a plurality of regions discriminated from one another by having different flow resistances. The liquid carrier drains through the regions of the forming belt according to and inversely proportional to the flow resistance presented thereby. For example, if there are impervious regions, such as protuberances or blockages in the forming belt, no liquid carrier can drain through these regions and hence relatively few or no fibers will be deposited in such regions.
The flow resistance of the forming belt according to the present invention is thus critical to determining the pattern in which the cellulosic fibers entrained in the liquid carrier will be deposited. Generally, more fibers will be deposited in zones of the forming belt having a relatively lesser flow resistance, because more liquid carrier may drain through such regions. However, it is to be recognized that the flow resistance of a particular region on the forming belt is not constant and will change as a function of time.
Such change occurs because as the cellulosic fibers are deposited onto a region of the forming belt the cellulosic fibers will obturate the region, increasing its flow resistance. Obturation and increased flow resistance in a region result in generally reducing the amount liquid carrier which drains therethrough and, hence, the amount of fibers later and further deposited onto this same region.